This invention relates to a hydraulic braking system, including a pair of mutually independent hydraulic circuits, for a vehicle. It relates, more particularly, to a hydraulic brake system provided with a controlling means which is capable of suitably adjusting the braking pressure of the wheel cylinders mounted on the rear wheels of a vehicle.
In many vehicle hydraulic brake systems the circuit connecting the master cylinder, wherein the braking hydraulic pressure is created, to the wheel cylinders, mounted on each front and rear wheel, is usually divided into two mutually independent circuits for the purpose of safety enhancement. Many of them are also provided with a pressure control valve or valves (hereinafter sometimes called simply control valve) within the circuit for the purpose of making an adjustment such that when the pressure in the master cylinder rises exceeding by chance a certain preset value the wheel cylinder pressure of the rear wheel (hereinafter called rear wheel cylinder pressure) is controlled so as to be smaller, in the rising rate, than the master cylinder pressure. The above adjustment is aimed at elimination of the lock phenomenon (wheels stop rotation inviting a slippage between the ground surface and the wheels themselves), which usually occurs in the rear wheels alone when the brake is applied, as well as attainment of the sufficient braking effect.
The abovementioned control valve is so set as to suitably determine, when the two braking hydraulic circuits are in good operating condition, the ratio of braking power between the front and rear wheels. When, however, a damage or trouble should occur in any one circuit, this control valve is liable to restrict the braking power at the rear wheel or wheels less than is necessary.
In the event of a trouble occurring in one circuit, the load decreasing, when the brake is applied, on the rear wheels is less than when both circuits are in normal condition, because the load shifting forward becomes less in such an occasion. So the braking power on the rear wheels can be increased, larger in comparison with the normal time, before the lock phenomenon begins to take place. When any one side circuit is damaged, decelerating rate of the vehicle is decreased in comparison with that in the normal condition wherein both circuits are in good order. In other words the control valve restricts the sufficient increase of the braking power on the rear wheels, when such a trouble occurs in one circuit out of the two. In such a situation reduction of load on the rear wheels owing to a forward shifting of load becomes less in comparison with the normal condition, because of decreasing of the deceleration rate. Braking pressure on the rear wheels should be larger than in the normal condition, so long as the locking is considered. Nevertheless, the control valve restricts the rear wheel cylinder pressure likewise in the normal condition, wherein both circuits are in good order. It means that the control valve makes it impossible to sufficiently raise the braking power as desired on the rear wheel or wheels.
There have conventionally been proposed, in case of a trouble happening in one circuit, various methods or steps for sufficiently raising the rear wheel braking power, such as by blocking or restricting the operation of the control valve, or by means of by-passing the hydraulic power independently of the control valve. There is one example, wherein on a part of a piston for controlling the hydraulic power within the control valve, a hydraulic power from a separate circuit is applied in the direction of closing the control valve, for enabling the control valve to function, in this status, as the ordinary control valve. (The circuit, the hydraulic pressure thereof being under the control of this control valve, is hereinafter called the controlled circuit. A separate circuit hereinafter called the circuit independent of the above circuit is provided, the hydraulic pressure thereof being led into the above mentioned control valve.) By means of such an arrangement it becomes possible, when the other circuit is damaged to be unable to sufficiently raise the pressure, to make the hydraulic pressure for actuating the piston insufficient or scanty. The control valve will consequently not start the controlling operation unless the controlled circuit is supplied with enough pressure for compensating the lessened pressure, which means a larger braking power than that in the normal condition (both circuits are in good order) on the rear wheel or wheels can be attained. This hydraulic control system is a very excellent device with a simplified construction, being capable of functioning suitably in two different ways, i.e., when both circuits are in normal condition and when any one circuit happens to be malfunctioning. Nevertheless, it still can not be said to be perfect; it can not match the preferred embodiment of this invention in case of one circuit being damaged, as described later in detail.